Some applications, such as liquid crystal display (LCD), are inherently dependent on linearly polarized light. The generation of polarized light is today mainly produced passively by filtering unpolarized light, which unavoidably reduces the device efficiency, or by the direct generation of polarized light by stimulated emission in laser devices.
The interest for group-III nitrides has grown rapidly during the last years. One reason is the semiconducting properties of group-III nitrides that may be utilised in light-emitting diodes (LED) and laser diodes (LD).
Semiconductor quantum dots (QDs) have an important role in various light-emitting diodes. For example, QDs incorporated in the active layer of a light emitting diode or a laser diode could improve their efficiency. Photons emitted from QDs posses specific energies that can be time-correlated and/or quantum-entangled and such single photon characteristics are promising for quantum cryptographic applications (QCA) and other quantum information applications (QIA).
In these applications, one QD represents an individual quantum light source and it is of importance that the QD can be positioned in a controllable manner in order to facilitate the subsequent device processing.
Pyramidal-structured templates which provide preferential formation locations for the QDs are a way of achieving site control of the QDs. The pyramids are achieved by putting a mask with holes in it over a grown layer and depositing a semiconductor material, which grows epitaxially on the grown layer, forming hexagonal pyramids. A layer of another semiconductor, denoted the active layer, is deposited over the pyramid and the active layer is in turn covered by the same material as the pyramid is made of. This active layer has a lower band gap than its surroundings, thus forming a quantum well (QW) and at the tip of the pyramid a quantum dot of the active layer is formed.
Sharp emission peaks are an evidence of three-dimensional quantum confinement with quantized energy levels of QDs. Single site-controlled QDs located at the apexes of hexagonal pyramids can give sharp emission lines. The emission energies can be tuned within a certain energy range by varying the growth temperature of the active layer in the pyramid. In addition, the emission lines of single QDs on various pyramids tend to be polarized in a certain direction.
This process has been shown to work for InGaN QDs on GaN pyramids grown in the <0001> direction with six equivalent faces of {1011} owing to the hexagonal wurtzite crystal symmetry of GaN. The characteristics of these single QDs and the ability to fabricate them in a controllable fashion show potential for the QDs as quantum light emitters (QLE).
The in-plane control of the light polarization is of great importance in a wide range of scientific and technological areas. Besides the solid state lightning applications, the fields of quantum cryptography are in need of a reliable single photon source which emits photons with deterministic polarization vectors. Such a polarization deterministic single photon source should preferably have a narrow spectral line width, have compatibility with modern electronics and allow room temperature operation.
To increase the usability of these QDs, it would also be preferred if the polarization direction of the emitted photons could be modified to include more directions and that those additional directions could be controllable. Thus, there exists a demand for an alternate direct or at least improved generation of polarized light. Consequently, there exists a demand for controlling the polarization direction of the emitted light in group-III nitride pyramid QDs.